This invention relates generally to a circuit for monitoring the power supply from which a microprocessor is energized and, more specifically, to a circuit for generating a signal which resets a microprocessor upon initial start-up of the power supply and whenever the supply voltage drops below a preselected level.
Microprocessors are designed to operate reliably only when a supply voltage of a preselected magnitude is continuously applied. Whenever the supply is initially powered up or when the supply drops below the preselected level the microprocessor must be reset to insure that it will operate predictably. Since the resetting process of the microprocessor requires a minimum time duration to be accomplished microprocessor manufacturers specify that the reset signal persist for a preselected minimum time duration in order to insure that bistable devices which make up the operating memory, registers and timing elements have sufficient time to settle into a known start-up condition.
Thus, it is an object of the invention to provide a reset signal generator circuit which insures that upon initial power up of the microprocessor a reset signal of a preselected duration is generated to allow for carrying out of the reset process. In addition, the circuit provides a reset signal in response to a drop in the power supply voltage below the preselected level, which reset signal is maintained for a minimum time irrespective of an abrupt return of the supply voltage to a value above the preselected level. Thus, notwithstanding a rapid power fluctuation, the required reset signal is applied to properly complete the resetting of the microprocessor.